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Dive into the research topics where Ji-Hee Kim is active.

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Featured researches published by Ji-Hee Kim.


Nano Letters | 2006

Coherent Lattice Vibrations in Single-Walled Carbon Nanotubes

Yong-Sik Lim; Ki-Ju Yee; Ji-Hee Kim; Erik Haroz; Jonah Shaver; Junichiro Kono; Stephen K. Doorn; Robert H. Hauge; Richard E. Smalley

We have generated and detected coherent lattice vibrations in single-walled carbon nanotubes corresponding to the radial breathing mode (RBM) using ultrashort laser pulses. Because the band gap is a function of diameter, these RBM-induced diameter oscillations cause ultrafast band gap oscillations, thereby modulating the interband excitonic resonances at the phonon frequencies (3-9 THz). Excitation spectra show a large number of pronounced peaks, allowing the determination of the chiralities present in particular samples and relative population differences of particular chiralities between samples.


Journal of Physics: Condensed Matter | 2013

Theory of coherent phonons in carbon nanotubes and graphene nanoribbons

G. D. Sanders; Ahmad R. T. Nugraha; Kentaro Sato; Ji-Hee Kim; Junichiro Kono; Riichiro Saito; Christopher J. Stanton

We survey our recent theoretical studies on the generation and detection of coherent radial breathing mode (RBM) phonons in single-walled carbon nanotubes and coherent radial breathing like mode (RBLM) phonons in graphene nanoribbons. We present a microscopic theory for the electronic states, phonon modes, optical matrix elements and electron-phonon interaction matrix elements that allows us to calculate the coherent phonon spectrum. An extended tight-binding (ETB) model has been used for the electronic structure and a valence force field (VFF) model has been used for the phonon modes. The coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on the photoexcited carrier density. We discuss the dependence of the coherent phonon spectrum on the nanotube chirality and type, and also on the graphene nanoribbon mod number and class (armchair versus zigzag). We compare these results with a simpler effective mass theory where reasonable agreement with the main features of the coherent phonon spectrum is found. In particular, the effective mass theory helps us to understand the initial phase of the coherent phonon oscillations for a given nanotube chirality and type. We compare these results to two different experiments for nanotubes: (i) micelle suspended tubes and (ii) aligned nanotube films. In the case of graphene nanoribbons, there are no experimental observations to date. We also discuss, based on the evaluation of the electron-phonon interaction matrix elements, the initial phase of the coherent phonon amplitude and its dependence on the chirality and type. Finally, we discuss previously unpublished results for coherent phonon amplitudes in zigzag nanoribbons obtained using an effective mass theory.


Nature Physics | 2012

Giant superfluorescent bursts from a semiconductor magneto-plasma

G. Timothy Noe; Ji-Hee Kim; Jinho Lee; Yongrui Wang; Aleksander K. Wójcik; Stephen McGill; D. H. Reitze; Alexey Belyanin; Junichiro Kono

Superfluorescence—the emission of coherent light from an initially incoherent collection of excited dipoles—is now identified in a semiconductor. Laser-excited electron–hole pairs spontaneously polarize and then abruptly decay to produce intense pulses of light.


New Journal of Physics | 2010

Ultrafast IR spectroscopic study of coherent phonons and dynamic spin–lattice coupling in multiferroic LuMnO3

Kyeong-Jin Jang; Jongseok Lim; Jaewook Ahn; Ji-Hee Kim; Ki-Ju Yee; Jai Seok Ahn; Sang-Wook Cheong

The concurrent existence of ferroelectricity and magnetism within a single crystalline system characterizes the multiferroic materials discovered in recent years. To understand and develop the multiferroic phenomenon, we need to investigate the unusual coupling between spin and lattice degrees of freedom. Spins in multiferroics are expected to be elastically coupled to phonons. Therefore, the time-dependent study can be a crucial factor in understanding the coupled dynamics. Here, we report the observations of strong dynamic spin–lattice coupling in multiferroic LuMnO3. A coherent optical phonon of 3.6 THz and its temperature dependence is measured for the first time from our femtosecond IR pump and probe spectroscopy. Also, we observed a coherent acoustic phonon of 47 GHz similar to a previous report (Lim et al 2003 Appl. Phys. Lett. 83 4800). Temperature-dependent measurements show that both optical and acoustic phonons become significantly underdamped as temperature decreases to TN, and they disappear below TN. These observations reveal that phonons are coupled to spins by magneto-elastic coupling, and the disappearance of phonon modes at TN is consistent with the isostructural coupling scheme suggested by Lee et al (2008 Nature 451 805).


Nano Letters | 2014

Ultrafast Generation of Fundamental and Multiple-Order Phonon Excitations in Highly Enriched (6,5) Single-Wall Carbon Nanotubes

Yong-Sik Lim; Ahmad R. T. Nugraha; Sung-Jae Cho; Min-Young Noh; Eunjin Yoon; Huaping Liu; Ji-Hee Kim; Hagen Telg; Erik Haroz; G. D. Sanders; Sung-Hoon Baik; Hiromichi Kataura; Stephen K. Doorn; Christopher J. Stanton; Riichiro Saito; Junichiro Kono; Taiha Joo

Using a macroscopic ensemble of highly enriched (6,5) single-wall carbon nanotubes, combined with high signal-to-noise ratio and time-dependent differential transmission spectroscopy, we have generated vibrational modes in an ultrawide spectral range (10-3000 cm(-1)). A total of 14 modes were clearly resolved and identified, including fundamental modes of A, E1, and E2 symmetries and their combinational modes involving two and three phonons. Through comparison with continuous wave Raman spectra as well as calculations based on an extended tight-binding model, we were able to identify all the observed peaks and determine the frequencies of the individual and combined modes. We provide a full summary of phonon frequencies for (6,5) nanotubes that can serve as a basic reference with which to refine our understanding of nanotube phonon spectra as well as a testbed for new theoretical models.


Archive | 2013

Single-Walled Carbon Nanotubes

Sebastien Nanot; Nicholas A. Thompson; Ji-Hee Kim; Xuan Wang; William D. Rice; Erik Haroz; Yogeeswaran Ganesan; Cary L. Pint; Junichiro Kono

Single-walled carbon nanotubes (SWCNTs) are hollow, long cylinders with extremely large aspect ratios, made of one atomic sheet of carbon atoms in a honeycomb lattice. They possess extraordinary thermal, mechanical, and electrical properties and are considered as one of the most promising nanomaterials for applications and basic research. This chapter describes the structural, electronic, vibrational, optical, transport, mechanical, and thermal properties of these unusual one-dimensional (1-D) nanomaterials. The crystallographic (Sect. 4.2.1), electronic (Sect. 4.2.2), vibrational (Sect. 4.2.3), optical (Sect. 4.4), transport (Sect. 4.5), thermal (Sect. 4.6.1), and mechanical (Sect. 4.6.2) properties of these unusual 1-D nanomaterials will be outlined. In addition, we will provide an overview of the various methods developed for synthesizing SWCNTs in Sect. 4.3.


ACS Nano | 2010

Resonant Coherent Phonon Generation in Single-Walled Carbon Nanotubes through Near-Band-Edge Excitation

Yong-Sik Lim; Jae-Geum Ahn; Ji-Hee Kim; Ki-Ju Yee; Taiha Joo; Sung-Hoon Baik; Erik Haroz; Layla G. Booshehri; Junichiro Kono

We have observed large-amplitude coherent phonon oscillations of radial breathing modes (RBMs) in single-walled carbon nanotubes excited through the lowest-energy (E(11)) interband transitions. In contrast to the previously studied coherent phonons excited through higher-energy (E(22)) transitions, these RBMs show comparable intensities between (n-m) mod 3 = +1 and -1 nanotubes. We also find the novel observation of RBMs excited over an excitation range of approximately 300 meV above the E(11) transition, which we attribute to possible resonance with phonon sidebands of the lowest optical transition, arising from strong exciton-phonon coupling.


Scientific Reports | 2013

Fermi-edge superfluorescence from a quantum-degenerate electron-hole gas

Ji-Hee Kim; G. Timothy Noe; Stephen McGill; Yongrui Wang; Aleksander K. Wójcik; Alexey Belyanin; Junichiro Kono

Nonequilibrium can be a source of order. This rather counterintuitive statement has been proven to be true through a variety of fluctuation-driven, self-organization behaviors exhibited by out-of-equilibrium, many-body systems in nature (physical, chemical, and biological), resulting in the spontaneous appearance of macroscopic coherence. Here, we report on the observation of spontaneous bursts of coherent radiation from a quantum-degenerate gas of nonequilibrium electron-hole pairs in semiconductor quantum wells. Unlike typical spontaneous emission from semiconductors, which occurs at the band edge, the observed emission occurs at the quasi-Fermi edge of the carrier distribution. As the carriers are consumed by recombination, the quasi-Fermi energy goes down toward the band edge, and we observe a continuously red-shifting streak. We interpret this emission as cooperative spontaneous recombination of electron-hole pairs, or superfluorescence (SF), which is enhanced by Coulomb interactions near the Fermi edge. This novel many-body enhancement allows the magnitude of the spontaneously developed macroscopic polarization to exceed the maximum value for ordinary SF, making electron-hole SF even more “super” than atomic SF.


Journal of Applied Physics | 2009

Polarization anisotropy of transient carrier and phonon dynamics in carbon nanotubes

Ji-Hee Kim; Jaegyu Park; Bong Yeon Lee; Donghan Lee; Ki-Ju Yee; Yong-Sik Lim; Layla G. Booshehri; Erik Haroz; Junichiro Kono; Sung-Hoon Baik

We report on polarization-dependent transient carrier dynamics and coherent phonon oscillations in single-walled carbon nanotubes by determining the relation between the nanotube axis and the incident light polarization. Due to the anisotropic shape of nanotubes, optical absorption strongly depends on the polarization direction. We observed three decay components when the excitation wavelength was resonant with the E22 transition energy and observed two-decay components under off-resonance conditions. The transient absorption and coherent phonon amplitudes were measured as a function of the angle between the pump and probe polarizations and were analyzed based on the absorption anisotropy of carbon nanotubes.


Optics Express | 2008

Synchronously pumped optical parametric oscillator based on periodically poled MgO-doped lithium niobate

Kang-Jeon Han; Dong-Wook Jang; Ji-Hee Kim; Chang-Ki Min; Taiha Joo; Yong-Sik Lim; Donghan Lee; Ki-Ju Yee

We demonstrate a room-temperature operation of the near-infrared femtosecond optical parametric oscillator based on MgO-doped stoichiometric periodically-poled lithium niobate, which is synchronously pumped by a Kerr-lens mode-locked Ti:sapphire laser. Wide tunability in the range from 0.98 microm to 1.50 microm was enabled for a single set of cavity mirrors by incorporating a BK7 window as an output coupler. For the output coupling ratio of 3.7%, the threshold pumping power of 460 mW and the slope power conversion efficiency of 37% were achieved. By controlling dispersion values with intra-cavity prisms, femtosecond pulses as short as 66 fs could be obtained.

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Ki-Ju Yee

Chungnam National University

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Stephen McGill

Florida State University

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Kang-Jeon Han

Chungnam National University

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